NSCA host puts service into UNKNOWN state, but client seems operational

[phijor]

We encountered an error where the the NSCA host displayed a service in state UNKNOWN since it did not receive check results for a long time. Nonetheless, the metricq-sink-nsca client seemed to run without any issue.
After restarting the client, the problemen vanished and the service state recovered.

This might have been caused by one the following:

• there is a bug in metricq-sink-nsca where it continues to consume metric data, but does not send any new reports.
• (unlikely after having a look at the NSCA host logs): metricq-sink-nsca was fully functional, successfully sending check results, but the NSCA host dropped them along the way

In the latter case, we should debug the problem by logging whenever a report was sent successfully. I released version 1.8.1 that includes more log messages when reports are sent.

TODO:

• actually figure out why the client refuses to send reports randomly